Method and apparatus for precision working of material

ABSTRACT

A method for precise working of material, particularly organic tissue, comprises the step of providing laser pulses with a pulse length between 50 fs and 1 ps and with a pulse frequency from 50 kHz to 1 MHz and with a wavelength between 600 and 2000 nm for acting on the material to be worked. Apparatus, in accordance with the invention, for precise working of material, particularly organic tissue comprising a pulsed laser, wherein the laser has a pulse length between 50 fs and 1 ps and with a pulse frequency of from 50 kHz to 1 MHz is also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation Application of application Ser. No.10/625,157, filed Jul. 23, 2003 now U.S. Pat. No. 7,351,241, whichclaims the priority of U.S. Provisional Application No. 60/475,583,filed Jun. 2, 2003, the complete disclosure of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The invention is directed to a femtosecond laser system for preciseworking of material and tissue, particularly a laser device for theprecise, micrometer-exact working of organic material, preferably aneye.

b) Description of the Related Art

In a valuable contribution to the art, German Patent DE 197 46 483 bythe present Applicant describes how macroscopic amounts of material areablated, vaporized or melted (CO₂ laser, Nd:YAG, excimer . . . ) withmicrometric precision when working over large surface areas of materialsby means of lasers with large spot diameters.

In another valuable contribution to the art, German Patent DE 197 27 573by the present Applicant describes an algorithm by which a laser beamcan be deflected in order to ensure the best possible, most preciseworking of material.

U.S. Pat. No. 5,656,186 describes a method for working material whilesimultaneously preventing or minimizing damaging side effects (meltingedges, thermal damage, acoustic shock waves, cracking) through theselection of a special pulse duration depending on the material.

The material-working action of the laser is limited to the small spatialarea of the laser focus (typically a few μm³) in which the lightintensity is high enough to exceed the threshold of optical breakdown.Localizing on the focus volume, the cohesion of the material isdestroyed and a cavitation bubble is formed. When the laser focus isdirected to a new position for every laser pulse, linear, flat orthree-dimensional cut patterns can be generated. The distance betweenadjacent cavitation bubbles at the conclusion of the operation mustapproximately correspond to their diameter so that the material caneasily be removed mechanically along the cuts.

Existing laser instruments for working materials with femtosecond laserpulses use regenerative amplifiers with repetition rates of up to 15 kHzby which individual pulses of a femtosecond oscillator are amplified.While the oscillator itself only provides pulse energies in thenanojoule range, the pulses can be amplified up to several millijoulesby a regenerative amplifier. While these laser sources are suitable forapplications with high ablation rates per laser pulse, they are notoptimal for the above-described application for precision cuts.

It is known to use lasers of the type mentioned above for refractivecornea surgery. Usual pulse energies are 5 μJ to 10 μJ. Cavitationbubbles with diameters of 10 μm to 30 μm are generated in this way.These bubble dimensions cause a micro-roughness of the generated cut onthe same order of magnitude. On the other hand, it is known that amicro-roughness on this order of magnitude allows only unsatisfactoryrefractive results.

K. König et al., Optics Letters, Vol. 26, No. 11 (2001) describes howcuts in tissue can also be carried out with nanojoule pulses from afemtosecond oscillator. However, due to the fact that an individuallaser pulse does not lead to formation of a cavitation bubble but that,rather, a plurality of pulses placed at the same location are needed togenerate a cutting action, this method is only suitable for very finecut shapes on the micrometer scale. This laser source is not suitablefor industrial or medical use.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, it is the primary object of the present invention to providean apparatus for precise working of material which overcomes thedisadvantages of the prior art.

In particular, the above-stated object is met by an apparatus forprecise working of material, particularly organic material. Thisapparatus generates cavitation bubbles with a diameter of less than 10μm in the material to be worked. In order to achieve this, a pulsedlaser beam with a pulse energy of less than 5 μJ is focused on a focusdiameter of a few micrometers. The focus diameter is preferably about 3μm and the pulse energy is preferably 1 μJ. Further, the apparatus ischaracterized in that it permits very fast working through the use of apulse repetition rate of greater than 50 kHz. This is a great advantageparticularly for refractive cornea surgery because an operating time ofa few seconds to about 1 minute is achieved in this way.

The object is further met by an apparatus for precise working ofmaterial, particularly organic material, comprising a pulsed lasersystem with the above-described parameters as radiation source in whicha work beam of the radiation source can be applied to the material bymeans of a beam device with at least one device for beam deflection. Thepulse emission is correlated with the beam deflection and the device forbeam deflection comprises means for enabling laser pulses. By enablingis meant that the laser is enabled for a laser pulse and the laser pulseis triggered as soon as the laser is again able to deliver a laser pulsecorresponding to its maximum repetition rate. When it is stated that thepulse emission is correlated with the beam deflection, this means inparticular that the pulse emission can be carried out when the beam hasbeen directed to a determined point; that is, the pulse emission iscontrolled depending on the beam deflection.

In a special construction, the above-stated object is met through anapparatus for precise working of material, particularly organicmaterial, comprising a pulsed laser system as radiation source. The beamenergy is about 100 nJ to 10 μJ, preferably 500 nJ to 5 μJ. Therepetition rate of the radiation is preferably 50 kHz to 1 MHz,particularly preferably 100 kHz to 500 kHz. The focus diameter of thebeam is preferably about 500 nm to 10 μm, particularly preferably 3 μmto 5 μm. The pulse duration of the radiation is preferably about 100 fsto 1 ps, particularly preferably 200 fs to 500 fs.

The arrangement for beam shaping and/or beam deflection or, generally,the beam-shaping and beam-deflection systems can comprise diffractive orrefractive microoptics or adaptive optics or conventional opticalsystems. A number of conventional optical elements can be replaced bydiffractive or refractive elements.

The apparatus mentioned above for precise working of material ispreferably used for opthalmologic eye treatment, particularly forcorrecting visual deficiency of an eye. The apparatus can be used to cuta flap or lenticle in the cornea for correcting visual deficiency. Inaddition to cutting the lenticle, the apparatus according to theinvention can also generate refractive structures in the cornea, forexample, in the form of flat spots placed side by side or a point cloud.

Laser shots can also be applied directly for generating refractivestructures. For example, small bubbles can be generated in the lens ofthe eye by evaporating material or liquid. A great many comparativelylow-energy laser shots such as can already be carried out with theapparatus according to the invention are required for this purpose.

It is likewise possible to introduce deliberate cuts in the tissue, forexample, the eye lens, with the apparatus according to the inventionand, therefore, to improve the flexibility and elasticity of the lens,since the adjacent tissue portions can now be displaced relative to oneanother more easily. The apparatus for precise working of material,particularly organic material, is used in this construction of theinvention as an apparatus for the treatment of presbyopia. The beamshaping is carried out either conventionally or with diffractive orrefractive microoptics or adaptive optics. The beam deflection ispreferably carried out by means of scanning systems.

Suitable laser beam sources are oscillator-amplifier arrangements.Suitable amplifiers are particularly regenerative amplifiers, chirpedpulse amplifiers (CPA) or multipass amplifiers.

With respect to the constructional form of the mode-coupled oscillator,disk laser oscillators, fiber laser oscillators, and rod laseroscillators are particularly suitable. As regards the constructionalform of the amplifier, disk laser amplifiers, fiber laser amplifiers,and rod laser amplifiers are particularly suitable.

Semiconductor laser diodes are particularly preferred as a pump sourcefor the laser media due to their long life, dependability,controllability and their comparatively low manufacturing costs.

Preferred laser media in the above-mentioned laser beam sources aredoped solid state materials, particularly crystals and glasses. Examplesare YAG, tungstate, sapphire and fluoride glass.

These host materials can preferably be doped with neodymium, erbium,titanium, chromium, lithium or ytterbium. All of these materials arecharacterized by a spectrally broadband laser emission in the spectralrange of 600 nm to 2000 nm and accordingly comprehend the spectralregion between 800 nm and 1200 nm which is particularly suitable forrefractive cornea surgery.

The large spectral bandwidth of the laser emission of theabove-mentioned materials supports an ultrashort laser pulse durationbetween 50 fs and 1 ps. In this respect, it is not necessary for thelaser itself to emit pulses of this pulse duration, but the preferredpulse duration of about 300 fs can be achieved in the workpiece to beworked or on the surface thereof. For this purpose, the apparatuscomprises an optical module which serves to influence the spectral phasefunction of the laser pulses in a suitable manner. In particular, thisoptical module generates a linear prechirp whose quantity is adapted tothe linear chirp of the optical system. This optical module can alreadybe suitably integrated in a laser beam source; in particular, it can becombined with or be identical to the pulse compressor of a CPA laserbeam source.

The material which is preferably to be worked with micrometer precisioncan comprise material with structures in the micrometer range, gratings,contact lenses, plastics, intraocular lenses (IOL), semiconductorwafers, microoptic elements, etc. Organic material such as tissue isparticularly preferred, especially the tissue of the human eye.

The pulsed laser system is an arrangement of a laser beam source forgenerating fs pulses and corresponding optical apparatus, particularlymirrors, lenses, etc.

In a construction of the apparatus according to the invention, the meansfor beam deflection are operated in scan mode. The work beam of theradiation source can be deflected on paths which recur periodically inone dimension, so that circular paths of different diameter or spiralpaths can be generated, for example. The paths of the work beam can begenerated by rotating apparatus or apparatus which can be kept on a pathin some other way, for example, by means of a mirror, a lens, a gratingor the like. The means for beam deflection can comprise scanners, e.g.,mechanical scanners, which are supported so as to be movable onpredetermined paths. The present invention uses fast deflection systemswhich deflect the laser on the natural paths of the deflection system,that is, e.g., on circular paths or spiral paths in rotating deflectionsystems. Instead of approaching individual positions and triggering alaser pulse there as soon as the given position is reached, whereuponthe deflection system is at rest again, the path of the deflectionsystem is traversed without stopping and the pulses are emittedbeginning at a defined time with a preselected repetition rate which isgiven by the path speed of the focusing movement.

As soon as the focus position has reached a determined point, the laseris enabled and laser pulses are accordingly sent to the work area. Thisresults in a track of working volumes, places in the material that aremodified by the laser focus during the short pulse duration, along asubstantially predefined path which is characterized in particular inthat adjacent working volumes are placed at uniform, predefineddistance, e.g., on the order of magnitude of the diameter of thecavitation bubbles. Additional tracks which together make up an incisionsurface can be written through a slight modification of the natural pathof the deflection system, e.g., through a slight reduction in thecircular path radius, for example, by an amount corresponding to thedistance between adjacent working volumes. For instance, concentricpaths, spiral paths or the like can be generated. When a deflectingmirror is used, this can be carried out, for example, by changing theinclination while maintaining the same rotation of the mirror. The aimis to cover the desired cut surface with a uniform grid of workingvolumes or laser foci. The natural paths of the deflection system can betraversed very quickly with a defined time sequence due to the highrepetition rate of the laser system. Adapting the time sequence of thelaser pulses then results in the desired coverage of the cut surfacewith laser shots.

Further, beam devices for beam shaping and/or beam control and/or beamdeflection and/or beam focusing are provided in another embodimentexample of the present invention. By means of these beam devices, thebeam can be directed and conducted to the material to be worked asprecisely as required in the intended application. Particularly becauseof their low pulse energy of about 1 μJ, the ultrashort laser pulseswhich are focused on a focus diameter on the order of magnitude of 3 μmcan undo the material coherence and/or bring about structural changes inthe material in a small, precise cavitation bubble without thermal,acoustic or mechanical loading of adjacent areas in the material. Formacroscopic cuts and structures in the centimeter scale, the laser focusis scanned in a three-dimensional manner through the material to beworked. The specific application determines how the beam source, beamcontrol and beam shaping, scanner, scan algorithm and focusing opticsare to be adapted to one another in order to achieve a high workingspeed with high precision at the same time.

The beam shaping is preferably carried out by means of a telescope(preferably a Galileo telescope with collecting lens and diverging lens)which expands the beam diameter in such a way that the laser can befocused on a corresponding small focus. A lens system which extensivelyminimizes the imaging error of the telescope is preferably used.

The beam control is preferably carried out by mirrors or pairs ofmirrors by which the beam can be adjusted in the individualsubcomponents.

The beam deflection arrangement can comprise conventional scanners ormechanical laser beam deflection systems such as galvanometer mirrors inclosed-loop mode, etc. However, mechanical scanners which travel givenpaths (e.g., circular paths) and are accordingly triggered by triggeringthe beam source at the provided positions are preferable. In this way, alarge area of the cut surface can be worked at full repetition rate withrelatively slow scanner movements.

The beam focusing device serves to cancel the coherence of the material(photodisruption) in the focus of the beam on or in the material. Thisis generally accompanied by local vaporization of the material. Thelaser is preferably focused on a diameter in the micrometer range forthis purpose. This is close to the diffraction limit of light in thevisible and near infrared range. Therefore, the focusing opticspreferably have a high numerical aperture and accordingly a short focallength and a large optical aperture (expanded laser beam diameter). Thebeam proceeding from the laser source is preferably expanded in diameterbefore focusing on the material or tissue. The systems for beam control,beam deflection and beam focusing are therefore preferably designed fora large beam diameter.

The laser source, beam deflection (scanner) and focusing optics are soadapted to one another that a precise, fast cutting is made possible byway of photodisruption. Laser spots with a focusing diameter of some 100nm to several micrometers are placed in the material with a spotdistance on the order of magnitude of the cavitation bubble diameter.

In a particularly preferred embodiment form, the beam devices,particularly the deflection devices, are programmable. Due to theability to adapt the individual beam devices to one another and tocontrol through corresponding programs, the system of beam devicestogether with the pulsed laser system can be adjusted exactly to thematerial, and cutting requirements for which it is to be employed.Accordingly, the set of parameters is preselected by the program andadapted depending upon the transparency and refractive power of thematerial to be worked and upon the required cut geometry and duration ofoperation.

In another preferred embodiment form of the present invention, holdingdevices are provided for positioning and/or fixating the material to beworked. These holding devices ensure that the micrometer-exactstructures which can be produced by the laser according to the inventionare not impaired by uncontrollable movements of the material to beworked, particularly the human eye.

A fixating and positioning device of the kind mentioned above can be asimple clamping device for a workpiece which is preferably outfittedwith multiaxial adjusting possibilities for moving and tilting theworkpiece for optimal adjustment. Further, fixating devices for medicalapplications in organs such as the eye must be adapted to the givenbiological factors. The human eye may be fixated, for example, by meansof a special adapter and a vacuum suction ring.

Because of the high repetition rates described above in conjunction withthe low pulse energies and the deflection devices described above, thelaser action for the photodisruption can be precisely localized. In thisway, the material structure can be destroyed in a sharply defined focusvolume; in closely adjacent areas (less than one micrometer apart),there is generally no change in the material. This results in highworking precision (micrometer accuracy) without damage to adjoiningregions of material. Thermal and mechanical loading of the regions thatare not worked is appreciably less than in other cutting methods.

In another preferred embodiment form of the apparatus according to theinvention, a work beam of the radiation source can be applied to thematerial by means of the beam device, particularly the deflectiondevice, in a geometrically predefinable shape in a time sequence thatcan be predetermined. Due to the interplay of the individual components,it is accordingly possible to generate cuts and structuring. One laserpulse with defined pulse parameters (pulse energy, pulse duration,focus) is generally sufficient to produce a spot in which the materialstructure is dissolved. A plurality of spots of this kind must be placedclose together for cuts and structuring. The distance between adjacentspots should be on the order of magnitude of the cavitation bubbles atthe conclusion of the procedure. For this purpose, the laser focus canbe moved in a scanning manner over and through the material. The laserfocus ideally follows a predetermined geometric path in athree-dimensional manner with micrometer accuracy. It is possible, forexample, to produce a cut in the material to be worked by moving in ascanning manner over any surface, e.g., a rectangular surface ofadjacent micrometer spots, in the tissue one after the other. As aresult, the material coherence is dissolved precisely in this plane anda cut is accordingly produced in the tissue. It is also possible toapply the laser focus to the material to be worked by circular movementsof the scanner in a circular path. By subsequently guiding the workingbeam in a helical pattern, a cylindrical surface can be cut out of thematerial, for example. Since the photodisruption preferably takes placewithin a very compact area, the laser beam can also act in tissuewithout damaging the material penetrated by the laser beam outside ofthe focus. In this way, any geometric paths, and therefore shapes, canbe cut out of the material though photodisruption.

In refractive cornea surgery, a special cut guiding can be realized bythe apparatus according to the invention. Rather than preparing atraditional flap, the lenticle which is prepared beforehand in thecornea by the apparatus according to the invention is extracted at thecircumference by means of one or more defined lateral cuts which arelikewise generated by the apparatus according to the invention. For thispurpose, it can be advantageous to split the lenticle beforehand by oneor more cuts with the apparatus according to the invention. Inparticular, splitting is useful when the parts are removed subsequentlyby suction with a suction/rinsing cannula.

In the preferred embodiment example of the present invention, anapparatus is provided in which the pulsed work beam can be applied tothe material by the beam deflection device, during which the repetitionrate of the pulses of the work beam can be modified. By providing adevice for modifying the repetition rate with simultaneous beam controlof the work beam over the material to be worked, a uniform pattern ofspots can be generated in an elegant manner on the material to beworked, even when the beam is directed to the material to be worked atdifferent angles or different speeds by the deflection device. Aparticularly striking advantage is achieved, for example, when thedeflection device directs the beam in circular paths to the material tobe worked and these circular paths are generated with a specialrotational frequency of the deflection device, particularly thedeflection mirror, for example. At a rotational frequency of 50 Hz, forexample, the laser beam is guided on a circular path of 1 cm diameter ata repetition rate of 300 kHz, then 60,000 spots are placed in auniformly distributed manner on every circular path per revolution. Whenthe beam is guided on a circle with a diameter of only 0.5 cm at thesame frequency of the deflection device, the same distance between theindividual spots as that produced when guiding the beam on the largercircular path can be produced on the material to be worked by loweringthe repetition rate of the pulsed work beam. By correspondinglymodifying the repetition rate depending on the geometry followed by thedeflection device, any geometric pattern can be generated on thematerial to be worked with an essentially constant distance betweenspots. For example, spirals can be traveled in which the repetition ratecontinuously decreases from the outside to the inside with a uniformrotational frequency of the deflection device. Apart from this, anyother geometric shapes are also possible. When a constant distancebetween the individual spots on the material is not intended but,rather, a higher spot density is to be achieved in a specific area and alower spot density is to be achieved in another area, this can also beproduced by a combination of the selected parameters for the repetitionrate of the work beam and the frequency or spatial course of thedeflection device. For example, it is also possible in a preferredmanner to generate gradually differing areas with different focusdensities. In the case of a circle, for example, the center can have avery small focus distance while the focus distance to the edge isincreasingly greater.

The object of the invention is also met through a method for applying fspulses of a laser beam source with the characteristics noted above,particularly high repetition rate and low pulse energy, to a material,particularly an organic material, in particular the human eye, in whichthe material is worked in the focus of the laser beam by means ofphotodisruption or its coherence is dissolved.

In a particularly preferred method of the present invention, the pulsedlaser beam is directed to the material to be worked by means of adeflection device and the repetition rate of the pulses of the laserbeam is modified depending on the spot pattern generated in this manneron the material. In this way, any spot pattern and particularly anydistance between the individual spots can be produced in the desiredgeometry on the material to be worked. In a particularly preferredmanner, the spot patterns are distributed on the material to beprocessed in such a way that the cavitation bubble of every individualspot occurring through photodisruption is placed exactly next to thecavitation bubble of the next spot. A desired cut pattern of directlyadjacent cavitation bubbles is formed in this way. For specialapplications, it can also be desirable to place the spots even closertogether. This is recommended, for example, when the material to beworked is renewed again after a certain time and the dissolution of thematerial is to be ensured for a specific time before, e.g., the drillcore or another piece that is cut out of the material to be worked canbe removed. It is also possible that the spots are initially placed at agreater distance from one another in order to fill the gaps between thespots in a subsequent step and accordingly to form a desired pattern ofcavitation bubbles.

The apparatus according to the invention can be used for refractivesurgery by working the cornea or the lens of the eye.

Additional advantageous constructions of the invention will be describedin the following with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a schematic view of an embodiment example of a laseraccording to the invention;

FIG. 2 shows another embodiment example of a laser according to theinvention with a surgical microscope and an eye to be worked;

FIG. 3 shows a schematic view of some examples of possible cut patternswhich can be made with the laser system according to the invention;

FIG. 4 shows a schematic detailed view of a sequence of laser spots oncircle lines; and

FIG. 5 shows the timeline of sequences of laser pulses in and outside ofthe laser resonator;

FIG. 6 shows the cutting control for generating a lenticle in sectionthrough the cornea;

FIG. 7 shows the process of extracting the cut lenticle through a smalllateral cut;

FIG. 8 shows the cut lenticle in a top view of the cornea;

FIG. 9 shows another form of the cutting control in which the lenticleis divided and can be extracted through two lateral cuts; and

FIG. 10 shows another embodiment of the method according to theinvention, wherein the lens is divided in many parts which are removedby a suction/rinsing device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic view of the individual components of anembodiment example of a laser system according to the invention. Theworking device 1 comprises an fs laser beam source as radiation source11. The laser beam 15 is coupled out to beam expansion optics 21 bymirrors and a beam splitter 57. The expanded laser beam 15′ is thenguided by a beam deflection device such as a scanner in XY-direction toa beam focusing device 24. The latter is displaceable in the Z-axis andaccordingly allows the displacement of the focus point by displacing thebeam focusing device along arrow Z. Alternatively, a focusing opticalsystem with variable focal length can be used in order to displace thefocus position in Z-direction in a controlled manner. In this way, thefocused laser spot 16 is directed onto the material 90 to be worked,which material 90 is held in its position by means of a fixating device32. In the present instance, the material 90 is a contact lens to beworked. The spot 16 can also be oriented by displacing the fixatingdevice 32 in direction XY′ or Z′ on or in the material.

The laser beam 15 generated by the radiation source 11 is focused on thematerial 90 by the working apparatus 1. A focus diameter of a fewmicrometers can be achieved in that the laser beam 15 is focused with abeam diameter of a few millimeters through optics with a few centimetersfocal length. For a Gaussian beam profile, for example, there is a focusdiameter of three micrometers when focusing a laser beam of wavelength1000 nm and a beam diameter of 10 mm with a focal length of 50 mm.

Generally, the diameter of the laser beam 15 at the output of theradiation source 11 is smaller than is necessary for optimal focusing.With beam expansion optics 21, the beam diameter can be adapted torequirements. A Galileo telescope (diverging lens plus collecting lens)which is adjusted to infinity can preferably be used as beam expansionoptics 21. There is no intermediate focus in this case which could leadto an optical breakdown in air under certain circumstances. Theremaining laser energy is accordingly higher and the beam profile isconsistently good. It is preferable to use lens systems which lead tooptimal imaging characteristics of the telescope. By adjusting thetelescope, manufacturing variations can also be compensated in the beamdivergence of the radiation source 11.

In this embodiment example, the laser focus is moved over or through thematerial in a scanning manner. The laser focus or laser spot 16 isaccordingly scanned three-dimensionally with micrometer accuracy. Theexpanded laser beam 15′ is deflected perpendicular to the original beamdirection by a deflection device 23. The position of the focus 16 afterthe focusing optics 24 is displaced perpendicular to the original beamdirection. The focus can accordingly be moved in a surface which isessentially plane and perpendicular to the laser beam direction (X/Ydirection). The movement parallel to the beam direction (Z-direction)can be carried out on one hand by moving the workpiece (see arrow Z′).The scan algorithms are then preferably configured in such a way thatthe workpiece need only be moved slowly and the fast scanning movementsare carried out by the deflecting unit. On the other hand, the focusingoptics can also be moved parallel to the laser beam direction (arrow Z)in order to lower the focus in the Z-direction. Particularly in medicalapplications, the second method is preferred because the patient cangenerally not be moved quickly enough.

The worked material 90 is fixated relative to the laser device in afixating and adjusting device 32. In this connection, the fixatingdevice is preferably adjusted vertical to and parallel to the beamdevice in order to be able to place the cut pattern at the intendedlocation in the material 90. A visible laser beam which proceeds from apilot laser 27 and is collinear with the working laser beam 15, 15′supports the adjustment.

Mirrors or pairs of mirrors 22 are provided for beam control and forprecision adjustment of the beam position between the individualcomponents. The mirrors are preferably so constituted that the workinglaser beam does not destroy the mirrors, but the mirrors are highlyreflecting for the wavelength of the working laser and are sufficientlyreflecting for the pilot laser. The coating is selected in such a waythat the mirror does not substantially lengthen the laser pulseduration. In a particularly preferable manner, at least one of themirrors is a chirped mirror with which the dispersion of all of theoptics present in the beam path can be compensated in order to achieveoptimally short pulses in the working focus.

FIG. 2 shows another embodiment example of the present laser workingapparatus with surgical microscope. The construction correspondsessentially to the construction in FIG. 1. Identical parts areidentified by the same reference numbers. In this example, a human eyeis provided as material 90. This laser device, with which precise cutscan be made in the cornea of the human eye, will be described in detailby way of example. A circular surface which follows the curvature of thecornea and is centered with respect to the optical axis of the eye is tobe cut inside the cornea with fs laser pulses. A cornea flap is formedfrom the circular surface to the outside of the cornea by an arc-shapededge cut and can be opened to the side after the laser cut.

A flap of the kind mentioned above is used to prepare for a LASIKoperation in which the thickness of the cornea is changed by laserablation in such a way that refractive errors of the eye arecompensated. Previously, this cut was carried out by a mechanicalkeratomy which requires a high level of training on the part of thephysician and is fraught with risk. In addition, a refractive correctionof the cornea can be carried out in the same work step through anothercurved circular surface which, together with the first circular surfaceof the flap, surrounds a lenticle that can be removed after opening theflap.

In the special embodiment of the invention, the eye is pressed by meansof a suction ring 32 against a contact glass 31 which is either plane orpreferably essentially adapted to the curvature of the cornea. Thesuction ring is fixedly connected with the outlet window of the laserdevice which provides for a defined position of the cornea relative tothe laser focus. The expanded femtosecond laser beam is focused in thecornea by optics 24. A beam splitter which is highly reflective for thelaser wavelength and transmits visible light reflects the laser beam inthe beam path of a surgical microscope which is used for observing andcentering the eye. The focusing optics 24 form a part of the microscopeobjective. Together with bundling optics, a real intermediate image ofthe cornea can be generated and can be observed three-dimensionally withthe stereo eyepiece 80. The beam deflection unit 23 deflects theexpanded laser beam 15 vertical to its propagation direction.Accordingly, the laser focus can be directed to different points in thecornea. The depth of focus can be varied by displacing the focusingoptics 23 along the optical axis or by adapting the focal length of thefocusing optics.

Circular paths are preferably traveled by the deflecting unit. Forcutting the circular surface, the circle radius is reduced from circularpath to circular path and the repetition rate is so adapted that auniform spot distance is maintained. The depth of focus is adapted fromcircular path to circular path in such a way that the cut follows thecurvature of the cornea. To perform astigmatic corrections of eyesight(cylindrical correction), the depth of focus can be moved up and downtwice over the course of the circular path, so that a lenticle with acylindrical lens portion is formed. For the flap edge, the depth offocus is slowly displaced from the base of the flap to the outside ofthe cornea while the radius remains fixed, so that a cylindrical jacketis formed. The laser beam must be interrupted on an arc-shaped segmentof the circles described above in order to leave a hinge at which theprepared flap is held. For this purpose, laser pulses are simply coupledout of the radiation source 11.

The radiation source 11 is a femtosecond radiation source with theparameters described above which is preferably directly diode-pumped andtherefore simple and reliable. The emitted laser beam 15 is preferablyexpanded to a 1- to 2-cm beam diameter by a Galileo telescope. A visiblelaser beam from a pilot laser 27 is superposed collinear to the expandedlaser beam 15 and is then scanned and focused together with the workinglaser beam. For this purpose, the beam splitter 57 is transparent forthe femtosecond laser wavelength and reflecting for the pilot beam.

The many possible cut shapes depend only on the scan algorithms. Inprinciple, a laser device such as that described for a great manyapplications (for example, for refractive correction of vision) in whichcuts or structural transformations are made within the transparent partsof the eye (cornea, lens, vitreous body) and on the nontransparent partssuch as the sclera, iris and cilliary body are suitable. Accordingly,the invention by far surpasses existing technologies in universality andprecision (avoidance of damage to surrounding tissue) even in this smallsub-area of the application.

Application examples of cut geometries which can be realized with thelaser system according to the invention are shown in FIGS. 3 a to 3 d.These applications are only given by way of example; any othergeometries can be realized. The coherence of the material 90 is canceled(photodisruption) in the focus 16 of the laser. In general, this isbrought about by a local vaporization of the material. After the actionof the laser pulse, the material structure is canceled within a smallvolume, the cavitation bubble (also referred to as spot 16 in thefollowing) permanently or for a period of time lasting at least untilthe end of the operating period. The use of a sharply focusedfemtosecond laser accordingly offers the most precise localization ofthe laser action. In the sharply defined focus volume, the materialstructure is accordingly destroyed while no change in the material takesplace in general in the closely adjacent areas (distance of less thanone micrometer). This results in a high working precision while avoidingdamage to neighboring regions of material.

For cuts and structuring, a large number of individual spots whichdissolve the material structure are placed close to one another. Thedistance between adjacent spots should be on the order of the spotdiameter at the end of the procedure. In FIG. 3 a, a predeterminedvolume (e.g., a bore hole in the material) is generated by completelyfilling the volume to be removed with individual spots 16. In anontransparent material of this kind, one proceeds by layers beginningwith the layer of spots facing the laser.

In FIG. 3 b, only the edge of the bore hole is covered by spots. In thiscase, a cut is shown through the material. The spots 16 should bearranged in a rotationally symmetric manner around the axis Z shown indashes. In this way, a drill core is generated in the middle of thematerial 90 to be worked. The drill core can be removed subsequently asa cohesive piece. The required quantity of laser pulses is accordinglyconsiderably reduced compared to FIG. 3 a particularly in largecross-sectional surfaces of the bore hole.

An undercut in a transparent material 90 is shown in FIG. 3 c. Since theradiation is not absorbed by the material 90, cohesive pieces ofmaterial can be detached from the material by placing the spots on theedge of the cut when this material adjoins the surface.

FIG. 3 d shows how voids or structures (e.g., changes in the opticalcharacteristics) can be generated depending upon the makeup of thematerial.

For macroscopic cut shapes (in the centimeter range), several millionlaser spots are required just to cover only the cut surface (FIGS. 3 band 3 c) with spots in a sufficient density. For many applications(particularly medical applications), it is advantageous to keep theworking time or treatment time as short as possible. Therefore,according to the invention, the radiation source of the laser device iscapable of delivering laser pulses with a high repetition rate. FIG. 4shows a schematic section of a possible scan pattern in which theindividual spots 16 worked by individual laser pulses are arranged alongpaths which can be traveled by the scanner in a continuous manner. Inorder to achieve a sufficiently great distance between spots at highrepetition rates of the radiation source 11, the focus is moved veryfast in at least one of three scanning dimensions. The scan algorithmsare therefore preferably designed in such a way that the spots areplaced along paths which correspond to the natural movements of thedeflection unit. The movement in the other two dimensions can then becarried out relatively slowly. The natural paths of the deflection unitcan be, e.g., circular paths which the deflection units can travel atfixed rotational frequencies. This can be carried out, e.g., by rotatingoptical elements in the deflection unit. The radius of the circular pathand the depth of focus (Z-direction) are then the gradually variablescan quantities. This variant is particularly suitable when rotationallysymmetric cut shapes must be generated. The repetition rate of the lasercan then be put to particularly effective use when the rotationalfrequency of the circular paths is selected in such a way that the fullrepetition rate of the radiation source leads to the desired spotdistance d in the largest circular paths (B) to be traveled. When thecircular paths are smaller in radius (A) when traveling over the cutpattern, the repetition rate of the source can be correspondinglyreduced, resulting again in the optimal spot distance. This adaptationof the repetition rate can easily be achieved with the laser radiationsource described above. Adapting the rotational frequency to therepetition rate of the source can be more difficult in technicalrespects, particularly when this is carried out continuously for everycircular path (A, B). However, for purposes of reducing the workingtime, it can be advantageous to adapt the rotational frequency to thesmaller circular paths in a few steps.

In FIG. 5, possible sequences of laser pulses are shown in and outsidean oscillator-amplifier arrangement. The rotational frequency of thelaser pulses in the oscillator 40 depend only on the resonator lengthand is predetermined for a determined radiation source and is around 100MHz with resonator lengths of a few meters. With the regenerativeamplification shown here, the pulses 41 are coupled into the amplifierand amplified, for example. When a lower repetition rate is desired, theamplification of the pulses 43 is carried out. The repetition rate ofthe amplified laser pulses is changed in an economical manner in thisway.

FIG. 6 shows a cut in the human cornea 107 with front side 100 and backside 101. The lenticle 103 is formed by the two surface cuts 104 and105. A small lateral cut 102 which leads as far as the front surface 100of the cornea makes it possible to extract the lenticle 103. Thisextraction is shown in FIG. 7. The remaining void collapses 106.

FIG. 8 shows the cornea in a top view. The border 111 of the lenticle103 and the cuts 102 leading to the front surface of the cornea areshown in the drawing. The front surface of the cornea is severed alongline 110 and makes it possible to extract the lens.

FIG. 9 shows another preferred form of making the cut. The lenticle isdivided into two parts 123 ad 124 by a cut 122. Instead of an individualextraction cut 110, two extraction cuts 120 and 121 are made.Subsequently, the lens part 123 is removed through the extraction cut120 and the lens part 124 is removed through the extraction cut 121.

FIG. 10 shows another expression of the method according to theinvention. In this case, the lenticle bordered by the edge 111 is cutinto many small fragments 132. These fragments 132 can now be sucked outby means of a cannula 133 which preferably has a diameter that isadapted to the fragment size. This process can be reinforced by arinsing device via a second cannula 134 which is introduced into anoppositely located duct or even the same duct. The rinsing medium 136,135 is preferably isotonic saline solution, but other solutions can alsobe used. This process achieves the minimum weakening of the corneathrough these methods of refractive laser surgery.

The invention was described with reference to preferred embodimentexamples. Further developments carried out by persons skilled in the artdo not constitute a departure from the protective scope defined by theclaims.

While the foregoing description and drawings represent the presentinvention, it will be obvious to those skilled in the art that variouschanges may be made therein without departing from the true spirit andscope of the present invention.

1. A method for precise working of material, particularly organictissue, comprising the step of providing laser pulses with a pulselength between 50 fs and 1 ps and with a pulse frequency greater than100 kHz, up to 500 kHz, and with a wavelength between 600 and 2000 nmfor acting on the material to be worked; wherein the laser pulses arefocused on or in the material and the focal points are guided in threedimensions; wherein the focus points are guided in such a way so as togenerate a first cut surface in the material; wherein the first cutsurface is a flat or three-dimensional cohesive cut surface; and whereina plurality of laser spots are placed close to one another at the edgeof the cut in the material, and the spots are arranged in a rotationallysymmetric manner around a z-axis.
 2. The method for precise working ofmaterial according to claim 1; wherein the z-axis is perpendicular to asurface of the material.
 3. The method for precise working of materialaccording to claim 1; wherein a second cut surface is generated in thematerial and, together with the first cut surface, surrounds anessentially lens-shaped severed portion of material.
 4. The method forprecise working of material according to claim 3; wherein additional cutsurfaces are generated in the severed portion of material.
 5. The methodfor precise working of material according to claim 3; wherein at leastone cut is generated between the material surface and the severedportion of material.
 6. The method for precise working of materialaccording to claim 5; wherein the severed portion of material isextracted from the material through the at least one cut.
 7. The methodfor precise working of material according to claim 6; wherein thematerial portion is divided into small fragments and the extraction ofthese fragments is carried out by a suction/rinsing device.
 8. Themethod for precise working of material according to claim 1; wherein theenergy of the individual pulses is between about 100 nJ and 5 μJ.
 9. Themethod for precise working of material according to claim 1; wherein thetime interval between the laser pulses is varied depending upon thelocation of the focus point.
 10. The method for precise working ofmaterial according to claim 1; wherein the speed at which the focuspoints are guided is varied depending upon the location of the focuspoints.
 11. A method for precise working of material, particularlyorganic tissue, comprising the step of providing laser pulses with apulse length between 50 fs and 1 ps and with a pulse frequency from 300kHz to 1 MHz and with a wavelength between 600 and 2000 nm for acting onthe material to be worked; wherein the laser pulses are focused on or inthe material and the focal points are guided in three dimensions;wherein the focus points are guided in such a way so as to generate afirst cut surface in the material; and wherein the first cut surface isa flat or three-dimensional cohesive cut surface.
 12. The method forprecise working of material according to claim 11; wherein the pulsefrequency is in the range of from 500 kHz to 1 MHz.
 13. The method forprecise working of material according to claim 11; wherein a second cutsurface is generated in the material and, together with the first cutsurface, surrounds an essentially lens-shaped severed portion ofmaterial.
 14. The method for precise working of material according toclaim 13; wherein additional cut surfaces are generated in the severedportion of material.
 15. The method for precise working of materialaccording to claim 13; wherein at least one cut is generated between thematerial surface and the severed portion of material.
 16. The method forprecise working of material according to claim 15; wherein the severedportion of material is extracted from the material through the at leastone cut.
 17. The method for precise working of material according toclaim 16; wherein the material portion is divided into small fragmentsand the extraction of these fragments is carried out by asuction/rinsing device.
 18. The method for precise working of materialaccording to claim 11; wherein the energy of the individual pulses isbetween about 100 nJ and 5 μJ.
 19. The method for precise working ofmaterial according to claim 11; wherein the time interval between thelaser pulses is varied depending upon the location of the focus point.20. The method for precise working of material according to claim 11;wherein the speed at which the focus points are guided is varieddepending upon the location of the focus points.